treat x* pattern arg as a Seq of the element type.

Author: zielinsky

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -382,7 +382,12 @@ object SpaceEngine {
       project(pat)
 
     case Typed(_, tpt) =>
-      Typ(erase(tpt.tpe.stripAnnots, isValue = true, isTyped = true), decomposed = false)
+      val tptTpe = 
+        if isWildcardStarArg(pat) then 
+          tpt.tpe.translateFromRepeated(toArray = false)
+        else 
+          tpt.tpe
+      Typ(erase(tptTpe.stripAnnots, isValue = true, isTyped = true), decomposed = false)
 
     case This(_) =>
       Typ(pat.tpe.stripAnnots, decomposed = false)

tests/warn/i23459.check

@@ -0,0 +1,6 @@
+-- [E092] Pattern Match Unchecked Warning: tests/warn/i23459.scala:7:14 ------------------------------------------------
+7 |    case Test(x*) => println(x) // warn
+  |              ^
+  |the type test for Seq[Int]* cannot be checked at runtime because its type arguments can't be determined from Seq[Int]
+  |
+  | longer explanation available when compiling with `-explain`

tests/warn/i23459.scala

@@ -0,0 +1,8 @@
+case class Test()
+object Test:
+  def unapply(t: Test): Some[Seq[Int]] = Some(Seq(1, 2))
+
+@main def run(): Unit =
+  Test() match
+    case Test(x*) => println(x) // warn
+